The present invention relates to the tomographic scanner and image reconstruction arts. The invention finds particular application in fourth generation computerized tomographic scanners and will be described with particular reference thereto.
Tomographic scanners have progressed from traverse and rotate early generation scanners to purely rotational later generation scanners. In one of the purely rotational tomographic scanners, commonly known as a third generation scanner, a source of fan beam radiation and an arc of radiation detectors rotate concurrently around an image region or scan circle. The third generation image reconstruction process assumes that the data in each view sampling results from concurrently occurring x-ray projections or rays. Each ray or projection extends from an apex at the x-ray tube focal spot to one of the detectors. The sampling of a fan of concurrent rays or projections with a common apex or position of the x-ray source constitutes a single view for the reconstruction process. Data collected in this fashion are commonly referred to as "source fan data".
In third generation scanners, the ray sampling is limited by the number of active detectors per view. The view sampling is limited by the data acquistion rate of the system. The faster data can be acquired, the more rapidly views can be sampled.
A fourth generation tomographic scanner has an annular array of detectors circumscribing the image region. A radiation fan beam source orbits around the image region to irradiate continuously shifting subsets of the stationary detector ring until the required amount of data is acquired. The image reconstruction algorithm uses each detector as the origin of a fan. Thus, the radiation rays or projections in each view or fan occur at different intervals in time as the source rotates to irradiate each detector from a plurality of directions. Rays are defined by the projections of radiation between the source and the active detectors during the periods of time when the outputs of the detectors are being sampled. Each detector is sampled a plurality of times as the radiation source sweeps opposite the image region from the detector to generate each view. The ray sampling is limited by the data acquisition rate of the system. View sampling is limited by the number of detectors in the ring.
The faithful reconstruction of images and the susceptibility to artifacts are trade-offs in both third and fourth generation scanners. Third generation scanners commonly have more detectors irradiated at a time. If ray samplings were performed in a fourth generation scanner in the same manner as in a third generation scanner, the resolution would be limited by the smaller number of detectors in the ring which are viewed at one time. A fourth generation scanner detector ring commonly contains about 1200 to 1500 detectors, with approximately 256 detectors viewed at any one time.
Increasing the number of detectors in the ring of fourth generation scanners could improve performance. However, increasing the number of detectors would increase the complexity of the system.
The present invention contemplates a new and improved tomographic scanner imaging modality. A fourth generation scanner is adapted to acquire data in a manner which is better suited for gated scanning. Source fan data is generated which has a ray sampling that is substantially equivalent to third generation scanners.